Wi the batteries show 85% at full charge?

I have an electronic cigarette (wape), there are 3 li-on batteries with 2700 mAh each. For about 2 months I charged them through the USB port, after this time, when fully charged, the cigarette defines them as charged with 85%. Last month it was defined as 92%. That is, there is a noticeable degradation.

Now the question is how to charge them correctly? I was told that you should use special chargers, and not charge cigarettes (USB) through the board. Is this really the case, or is battery degradation a normal phenomenon?